There are a number of methods for improving quality of communication in the digital wireless transmission such as an error-correction coding, a diversity transmission and reception, and a combination of them. A well-known method of error-correction coding among the above is a convolution coding having a superior performance in the error correction. One example is a newly devised method which combines interleaving and puncturing with the convolution coding, as disclosed in Japanese Patent Laid-Open Publication No. H08-298466. A method of the prior art for improving quality of communication using a combination of the convolution coding, the puncturing and the time-diversity will be briefly described by referring to FIGS. 8A and 8B.
In a transmission device 800 of FIG. 8A, a series of information data 851 to be transmitted is punctured (thinning-out process) in a unit of a fixed amount of data block by a puncturing unit 802 in order to reduce an amount of communication traffic in a transmission pathway, after it is convolution-coded by a convolutional coding unit 801. A puncturing (thinning-out) location within the data block is stored as a puncturing pattern in a puncturing pattern generator 803, from where it is supplied to the puncturing unit 802.
An example shown in FIG. 8B will be described now in detail. A series of input information data {a0, b0, c0, d0, . . . } is converted into a series of convolution-coded data {a1, a2, b1, b2, c1, c2, d1, d2, . . . } by the convolutional coding unit 801 having a constraint length of 3 and a coding rate of 1/2. The puncturing unit 802 removes b2, d1, etc., and outputs a series of punctured data {a1, a2, b1, c1, c2, d2, . . . }, when a puncturing pattern 803b is supplied from the puncturing pattern generator 803. This series of punctured data is a combination of a series of data {a1, b1, c1, e1, . . . }, which is obtained by deleting data corresponding to a0 position in an upper row of the puncturing pattern 803b from a series of data {a1, b1, c1, d1, e1, . . . } corresponding to the upper row of the puncturing pattern 803b out of the foregoing series of convolution-coded data, and another series of data {a2, c2, d2, e2, . . . }, which is obtained by deleting data corresponding to a0 position in a lower row of the puncturing pattern 803b from a series of data {a2, b2, c2, d2, e2, . . . } corresponding to the lower row of the puncturing pattern 803b out of the series of convolution-coded data.
A time-diversity modulator/transmitter 804 repeats modulation and transmission of the series of punctured data for a predetermined number of times in response to a diversity transmission timing control signal supplied by a diversity transmission timing controller 805 at intervals of a predetermined time.
In a receiving device 810, the predetermined time for the transmission device 800 to repeat the time-diversity transmission is set in advance with a diversity reception timing controller 811, so that the diversity reception timing controller 811 outputs a timing control signal for starting a time-diversity reception according to the set time. A time-diversity receiver/demodulator 812 receives and demodulates a signal transmitted repeatedly in response to the control signal of a time-diversity reception timing, and outputs a series of demodulated data of every diversity branch (every repeat time) . In this example, description is being made on an assumption that a result of demodulation for each symbol in the series of demodulated data is a quantized data in a resolution of four bits, and a mark and a space have their respective values equivalent to −7 and +7 under the condition of no influence of noises.
A puncturing pattern generator 813 stores a puncturing pattern, which is identical to the puncturing pattern 803b used in the puncturing unit 802 of the transmission device. A depuncturing unit 814 uses this puncturing pattern to depuncture the series of demodulated data of every diversity branch, and outputs a series of depunctured data of every diversity branch. The depuncturing is a process in which the punctured position is filled with a dummy data such as a value of 0 corresponding to a middle value between the soft decision value of −7 corresponding to a mark and the soft decision value of +7 corresponding to a space, for example. In the case of the foregoing series of punctured data {a1, a2, b1, c1, c2, d2, . . . }, the depuncturing unit 814 outputs a series of depunctured data {a1, a2, b1, 0, c1, c2, 0, d2, . . . }.
The series of depunctured data of every diversity branch obtained here is combineed symbol by symbol in a unit of block by a combining unit 815, and they are convolution-decoded with a method such as the Viterbi soft quantization by a convolutional decoding unit 816, which in turn outputs a series of decoded information data. There may be a case where the depuncturing and the combining are reversed in their order of transaction.
The devices can thus achieve an improvement in quality of communication for both of the error-correction coding and diversity with the structure as described above, by performing punctured-convolution-coding and time-diversity transmission on the information data to be transmitted, and also combining and depunctured-convolution-decoding after time-diversity reception of the data at the receiving side.
However, the structure of FIGS. 8A and 8B punctures certain identical locations in the series of convolution-coded data (error-correction code word) in each of the repeated transmissions by way of the time-diversity transmission. Therefore, these certain punctured locations and vicinity of them become susceptible to noises, as they become low in likelihood when convolution-decoding them, since they are treated as values having a large length between codes from both of the mark and the space at the receiving side.